← Glossary
Authentication
What is TLS / STARTTLS?
Encryption of the SMTP connection between mail servers.
How it works in practice
STARTTLS upgrades a plain SMTP connection to an encrypted one. Gmail requires TLS for bulk senders and shows the TLS status of every message, so a sending service that connects without it is a deliverability problem DNS records can't reveal.
The common mistake
Verifying SPF, DKIM and DMARC and assuming the connection is encrypted too.
Check it yourself
Related terms
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